Update: Took the trailer on its maiden voyage, ~600 miles round trip from Tallahassee to Orlando to Lakeland, and back to Tallahassee. I started off with a 100% grille block in the bottom and 100% on the top, and slowly scaled back until I found the fan didn't need to come on, which ended up being ~25% on the lower and 100% on the upper. Even with a 75% block on the bottom and 100% on top I was able to pull the trailer on the highway over moderate hills, but temps went up to ~96-97c on larger inclines. The bearings on the trailer didn't get warm with ~100lbs of cargo.
With an outside air temperature of around 0-5c (32-40f), and an intake temperature of 10-14c (50-60f), I found I was able to maintain lean burn at ~60mph with the trailer, and averaged between 65 and 68mpg on flat, level roads, depending on whether there was any air corridor effect. With similar driving in slightly warmer conditions, I have seen 74-78mpg without the trailer.
On the way home I increased my speed to ~70-75mph, occasionally hitting ~80 on downhills. In order to maintain these speeds, I had to keep my clutch switch engaged and was near WoT in 5th, which ended up being about ~49-50mpg. It's been a while since I drove at that speed, but in warmer weather I've seen as high as 70mpg at 70mph with intermittent A/C use, and my first trip with the car (before grille blocks and belly pan repairs) saw closer to 60mpg at that speed, so the aero penalty is definitely more noticeable at higher speeds.
When I arrived home, I found this:
My nose cone seems to have slid down and bowed in from the wind, which suggests to me it's not entirely out of the airstream. At 60mph it was fine, but it appears if I want to drive any faster, I'll need some small nails or staples to aid in keeping it in place. Cowmeat suggested adding some coroplast fairings attached to the back of the car, and I'm considering some small eye screws as mount points on the trailing edge of the hatch. I'm thinking one sheet on the top should be all that's necessary, as the trailer is narrow enough that the air should be converging somewhere behind the nose on the sides.